There is a list of extra balls at the bottom of the screen like tennis balls etc. I have collected enough points for the tennis ball, bit the icon for it is still locked. How are you meant to get additional balls in this game?
You need to click on the button on the top right of the screen to collect the points that you have earned so far. You may need to watch an advert before you are able to redeem the points. Once the points are added to your total above the level progress bar you will be able to spend the points to purchase new balls.